Infection of cells by HIV-1 is initiated by fusion of the viral and cellular membranes, a process mediated by the envelope glycoprotein on the surface of the virion. The HIV-1 envelope glycoprotein, gp160, is posttranslationally cleaved into two noncovalently associated subunits, gp120 and gp41 (1). The envelope glycoprotein complex consists of a trimer of three gp120 surface subunits and three gp41 transmembrane subunits. Analysis of its structure by X-ray crystallography shows that the NH2-terminal region of the ectodomain of gp41 forms a central, three-stranded coiled coil that is wrapped by an outer layer of three COOH-terminal helices in an antiparallel orientation around the outside of the coiled coil (6).
Each gp41 chain is anchored at its COOH terminus in the viral membrane. gp 120 determines viral tropism by binding to the cellular receptors, CD4 and members of the chemokine receptor family, whereas gp41 is responsible for fusing the viral and host cell membranes (2). There is substantial evidence to indicate that receptor binding triggers conformational changes in the gp120/gp41 complex, leading to activation of gp41 membrane fusion properties and ultimately invasion of the viral genome (3). By analogy with the ‘spring-loaded’ model of the influenza virus hemagglutinin (4), gp41 activation is postulated to involve a complex set of structural changes from a native (prefusogenic) state to a fusion-active (fusogenic) conformation (5).
Because the membrane anchor and the NH2-terminal fusion peptide of gp41 are embedded in the viral and target cell membranes respectively, formation of the trimer-of-hairpins structure is thought to appose two membranes for fusion (7). A monoclonal antibody recognizing this gp41 core binds specifically to the surface of HIV-1 infected cells only after addition of soluble CD4 (8). This observation provides direct evidence that the trimer-of-hairpins structure represents the fusion-active state of gp41 (6). The structure of the HIV-1 envelope glycoprotein in its native conformation is unknown.
The viral envelope glycoprotein that mediates HIV-1 entry into target cells is an important target for humoral immunity during the natural course of infection (30). Current recombinant gp120 or gp41 protein vaccine candidates are unable to elicit antibodies capable of neutralizing primary HIV-1 isolates from infected persons at sigpificant titers (34). These antibodies bind well to the individual gp120 and gp41 subunits but poorly to the membrane-associated, native envelope glycoprotein complex (FIG. 1) (35). In contrast, three infected human-derived neutralizing antibodies have been shown to recognize the native gp120/gp41 complex efficiently (10). Thus, it appears that an immune response to virions, rather than to nonnative forms of the envelope glycoprotein (called viral debris), leads to the production of functional antibodies. Since gp120 is readily dissociated or shed from gp41 (37), the challenge of inducing protective humoral immunity is to preserve the native envelope structure, or crucial components thereof, in vaccine preparations.
Because inhibition of gp41 -mediated membrane fusion has potential to offer a general strategy for the treatment or prevention of HIV-1 infection (9), structural information on the conformational change of the protein is crucial to guiding efforts to target this process for vaccine and antiviral drug development. In accordance with the present invention, it has been discovered that a peptide fragment derived from the COOH-terminal region of the gp41 ectodomain, forms a parallel three-stranded, α-helical coiled coil and serves as a trimerization domain in the native gp120/gp41 complex. A stable form of the trimeric coiled-coil domain elicits a strong antiviral antibody response against HIV-1 primary isolates from AIDS patients and thus has utility as a safe and practical HIV-1 vaccine immunogen.